Effect of Ca²⁺ ion co-doping on radiative properties via tuning the local symmetry around the Eu³⁺ ions in orange red light emitting GdPO₄:Eu³⁺ phosphors

Abstract

A series of Ca²⁺ substituted GdPO₄:Eu³⁺ novel phosphors were prepared via the solid state method. Rietveld refinement of the XRD data and transmission electron microscopy results confirmed that all these compounds adopted the monazite phase with space group P21/n. Fourier transform infrared spectroscopy (FTIR) analysis confirmed the presence of the characteristic vibrational bands for host matrix of GdPO₄, and field emission scanning electron microscopy (FESEM) results revealed that the particles possess agglomerated granular morphology. Photo-luminescent spectra displayed the representative luminescence ⁵D₀ → ⁷F_J (J = 0–4) intra-4f shell Eu³⁺ ion transitions. The magnetic dipole (⁵D₀ → ⁷F₁) transition was stronger than the electric dipole (⁵D₀ → ⁷F₂) transition. On co-doping Ca²⁺ content to Gd_0.93Eu_0.07PO₄ phosphor, we observed the enhanced PL intensity as a function of Ca²⁺ content; the maximum intensity of 1.5 times was observed for 7 mol% of calcium doped Gd_0.93Eu_0.07PO₄ phosphor. The enhancement of the PL intensity owing to the effective ionic radius and the mismatch in Pauling's electronegativity between co-dopant and host cations causes distortion of local crystal field surrounding the Eu³⁺ ions. Furthermore, this local distortion or reduced symmetry effect around the Eu³⁺ ions was reconfirmed by the Judd–Ofelt and life time decay analyses. The evaluated 1931 Commission International de l’Eclairage (CIE) chromaticity color coordinates exhibit orange red emission (x = 0.6247, y = 0.3748) with minimal CCT values and high color purity. This class of phosphors possessed excellent thermal stability at high temperature, and the integrated emission intensity at 423 K was about 66% of that at 303 K. Considering the above results, the Eu³⁺/Ca²⁺ co-doped GdPO₄ phosphors have potential applications in the near-UV excited white light emitting diodes.

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1. Introduction

For the past few years, astonishing transformation has been taking place in the electronics industry owing to its compactness, cost effectiveness, safety, energy conservation and environmental friendliness. Specifically, the light emitting diodes (LEDs), which are considered to be the future-generation solid-state lighting sources, have created their own value in the market due to their advantages such as reducing the usage of electricity for illumination, low-carbon emission, long life time, eco-friendliness, high brightness and so on. In particular, white light emitting diodes (WLEDs) have gained more attention due to their prominent advantages in the lighting industry.^1–7

The phosphor originated WLEDs realise the white color through the characteristic emission of phosphor materials.^8,9 In fabrication of WLEDs, there are mainly two approaches. The first one is the blend of a blue InGaN LED (450–470 nm) chip and Ce³⁺ activated Y₃Al₅O₁₂ (YAG) yellow phosphor distributed over a broad spectra to generate cool white light.¹⁰ However, this approach has some severe disadvantages such as these WLEDs showing a low colour rendering index (CRI < 80) and a high correlated colour temperature (more than 6000 K) due to the lack of red emission, which limits the application of these WLEDs in house hold and other practical applications.^11–13 To overcome these limitations, the second approach of fabrication of WLEDs, i.e., combination of tri-colour (red, blue and green) phosphors on NUV chip, was popularized owing to its color stability and excellent CRI, which made these WLEDs warm light sources, more like the natural light.^14–17 Unfortunately, the chemical stability and the efficiency of commercially available red phosphors (Y₂O₂S:Eu³⁺) are much lower compared to the commercialized blue (BaMgAl₁₀O₁₇:Eu²⁺) and green (ZnS:Cu⁺, Al³⁺) phosphors.^18,19 Hence, to achieve the highly efficient warm white light production, design of efficient, high color purity and low CCT red light emitting phosphors is essential. In order to obtain the efficiency and long durability of phosphor, selection of proper host and activator is very important.

Among the phosphate containing phosphors, gadolinium orthophosphate (GdPO₄) is an excellent host matrix, owing to its high optical damage threshold, large band gap, high absorption in the UV region, lower toxicity and excellent thermal and charge stabilization properties for rare earth ions. Moreover, phosphates can facilitate high crystal field environment, which greatly affects the rare earth ions’ characteristic emission.^20,21

Eu³⁺ activated GdPO₄ phosphor system received much attention in view of bio-medical and opto-electronic applications due to its inspirational luminescent properties.^20,21 From previous reports, it is evident that Europium activated GdPO₄ commonly crystallizes in the hexagonal structure at low temperature and crystallizes in monoclinic structure at high temperature. The monoclinic phase shows enhanced luminescent properties compared to those of its hexagonal structure.²² However, europium sources are very expensive and less abundant. Also, concentration quenching is an active phenomenon in rare earth doped phosphors above the threshold concentration of rare earth ions with respect to the host matrix. These problems kept out the europium activated phosphors from cost effective solid state lighting applications. Previously, Halappa et al.²² reported concentration quenching observed beyond 9 mole% of Eu³⁺ doping in GdPO₄ system.²² Evidently, it will be attractive if further enhancement in their luminescent properties with minimal rare earth ion concentration in the system for real time applications was possible.

Luminescent emission can be improved by modifying the phosphor composition, i.e. by co-doping small quantity of impurity ions such as Ca²⁺, Zn²⁺, and Li⁺ ions.^22–25 Generally, co-dopants enhance the luminescent intensity of the phosphors by modifying the crystal field environment or creating defects.^24,26 Among various co-dopants, Li⁺ has been extensively studied to enhance the luminescent efficiency of phosphors.^27,28 However, not much attention has been paid on the effect and the mechanism of Ca²⁺ ion co-doping in phosphors. To the best of our knowledge, the enhancement of luminescent intensity from Ca²⁺ ions in GdPO₄ phosphors has not been evaluated so far. Hence, Ca²⁺ is a motivational candidate for co-doping in Eu³⁺ activated GdPO₄ phosphor and investigates the effect of co-doping on luminescence properties. In this study, Eu³⁺/Ca²⁺ co-doped GdPO₄ phosphors have been prepared by a conventional solid state method. The structural and morphological studies were done using X-ray diffraction (XRD), transmission electron microscopy and scanning electron microscopy (SEM) techniques. The vibrational properties of the prepared phosphors were evaluated by Fourier transform infrared (FTIR) spectroscopy, and the effect of Ca²⁺ co-doping on luminescent and other radiative properties of Eu³⁺ activated GdPO₄ phosphor was investigated.

2. Experimental

2.1. Materials

All the chemicals purchased are of analytical reagent grade and used without further purification. In this preparation, gadolinium oxide (Gd₂O₃), di-ammonium hydrogen phosphate (NH₄)₂HPO₄, europium oxide (Eu₂O₃) and calcium carbonate (CaCO₃) were used as raw materials.

2.2. Synthesis

Series of Eu³⁺/Ca²⁺ co-doped GdPO₄ phosphors were synthesized by the solid state method. For the typical preparation of Gd_0.93Eu_0.07PO₄ phosphor, the requisite amounts of Gd₂O₃, Eu₂O₃ and (NH₄)₂HPO₄ were weighed and transferred to an agate mortar and ground well. The resultant mixture was kept in a porcelain crucible, and phase evaluation study of Eu³⁺ activated GdPO₄ phosphor was monitored by varying the calcination temperature from 650 °C to 950 °C for 12 h. Similarly, series of Ca²⁺ co-doped Eu³⁺ activated GdPO₄ phosphors were are also synthesized. The obtained powder product was used for further studies.

2.3. Characterization

The phase purity of the phosphor samples was evaluated by the powder X-ray diffraction (PXRD) technique on PANalytical Empyrean powder diffractometer with Cu K_α source (λ = 1.5418 Å). Rietveld refinement analysis was carried out using FullProf Suite programme. The vibration bands were identified using Perkin Elmer FTIR Spectrometer using KBr as a reference. The surface morphologies of the synthesized samples were investigated using field emission scanning electron microscopy (FE-SEM, FEI Quanta 200) with instrument operated at an accelerating voltage of 10 kV. Transmission electron microscopy (TEM) and high-resolution TEM images were recorded using JEOL 2100F/FEI Tecnai 200 KV. The photo-luminescent emission and excitation spectra of these phosphor powders were measured using a Jobin-Yvon Fluoro-Max spectro-fluorometer (with 450 W Xenon lamp source). The photo-luminescent decay measurements were recorded using Edinburgh Nano-second time resolved fluorescence spectrometer (equipped with λ_ex = 394 nm source). The temperature-dependent PL spectra of the samples were measured using Edinburgh FS5 spectro-fluorometer equipped with a temperature controller.

3. Results and discussion

3.1. Powder X-ray diffraction (PXRD)

Fig. S1 (ESI†) illustrates the powder X-ray diffraction (PXRD) patterns of Gd_0.93Eu_0.07PO₄ phosphor synthesized at 650 °C, 750 °C, 850 °C and 950 °C for 12 h, respectively. From Fig. S1 (ESI†) it is noticeable that PXRD pattern of the sample, calcined at 650 °C and 750 °C, shows peaks which correspond to the crystalline cubic Gd₂O₃ rich phase with a minor gadolinium phosphate phase. As the calcination temperature increased from 750 °C to 850 °C, the monoclinic GdPO₄ phase was enhanced with a minor cubic Gd₂O₃ phase. When calcination temperature was increased to 950 °C, the PXRD pattern of the sample matched well with monazite phase of GdPO₄ (space group P12₁/n₁, JCPDS card no. 84-0920) without any additional peaks of raw materials. This result indicates that 950 °C is the optimum calcination temperature to prepare Eu³⁺/Ca²⁺ co-doped GdPO₄ phosphor.

Fig. 1 represents the PXRD patterns of the Gd_0.93−xEu_0.07Ca_xPO₄ (0.00 ≤ x ≤ 0.07) phosphors prepared at 950 °C in 12 h. From Fig. 1, it is evident that all the X-ray diffracted peaks are in good agreement with the monazite phase of GdPO₄ with the space group P12₁/n₁ (JCPDS card no. 84-0920), and other characteristic peaks of impurity phases were not observed. This result indicates that substitution of Eu³⁺ and Ca²⁺ did not have any significant effect on monazite crystal structure of GdPO₄ host matrix.

Fig. 1 PXRD patterns of Gd_0.93−xEu_0.07Ca_xPO₄ phosphors calcined at 950 °C for 12 h.

The structural parameters of prepared phosphors were estimated using the Rietveld refinement method. In this study, the Rietveld refinement was executed using the FULLPROF Suite program.²⁹ The pseudo voigt function was used to fit numerous parameters. Observed, calculated and the difference XRD patterns of Gd_0.93−xEu_0.07Ca_xPO₄, (a) x = 0.00, (b) x = 0.01, (c) x = 0.03, (d) x = 0.05 and (e) x = 0.07 are shown in Fig. 2, and there is a decent agreement between the observed and the calculated patterns. The refined structural parameters for all the phosphors are summarized in Table S1 (ESI†). We did not observe any considerable changes in lattice parameters and cell volume on doping and co-doping of Eu³⁺/Ca²⁺ in the GdPO₄ host because of the comparable effective ionic radii of nine coordinated Gd³⁺ (1.107 Å), Eu³⁺ (1.120 Å), and Ca²⁺ (1.18 Å) ions.

Fig. 2 Observed, calculated and the difference XRD patterns of Gd_0.93−xEu_0.07Ca_xPO₄ phosphors: (a) x = 0.00, (b) x = 0.01, (c) x = 0.03, (d) x = 0.05 and (e) x = 0.07.

3.2. Electron microscopic analysis

Field emission scanning electron micrographic (FE-SEM) images of the solid state synthesized Gd_0.86Eu_0.07Ca_0.07PO₄ phosphors are displayed in Fig. 3a. It is worth noting that these samples exhibit agglomerated spherical shape particles with the average particle diameter varying from 0.25 to 0.40 μm. Fig. 3b–d display the Bright field TEM image, high resolution TEM image and SAED pattern of Gd_0.86Eu_0.07Ca_0.07PO₄ microcrystals, respectively. In Fig. 3c, the clear lattice fringes with lattice spacing of 0.235 nm correspond to the (112) plane of monoclinic GdPO₄ phase, reflecting the highly crystalline nature of the phosphor. Fig. 3d illustrates the indexed SAED image, which corresponds to the (101), (011), (200), (012), (120) and (−212) planes of monoclinic GdPO₄ phosphor and reconfirms the phase purity of the phosphor. Fig. S2 (ESI†) illustrates the Energy Dispersive spectra (EDS) of Gd_0.86Eu_0.07Ca_0.07PO₄ phosphor. From Fig. S2 (ESI†), it is evident that the phosphor is composed only of Gd, P, O, Eu and Ca elements and we did not observe any other impurity ions present in the phosphor. The presence of Eu and Ca elemental lines in the EDS confirms the doping of these ions in GdPO₄ matrix.

Fig. 3 (a) FE-SEM image, (b) bright field TEM image, (c) HRTEM image, (d) SAED pattern of Gd_0.86Eu_0.07Ca_0.07PO₄ phosphors.

3.3. Fourier transform infrared (FTIR) studies

Fig. 4 illustrates the FT-IR spectra of Gd_0.93−xEu_0.07Ca_xPO₄ (0.0 ≤ x ≤ 0.07) phosphors, recorded in the range from 400 to 1400 cm⁻¹. A prominent absorption band, observed at 966 cm⁻¹, corresponds to the symmetric stretching of P–O bonds in PO₄³⁻ tetrahedra. The bands situated at 543, 574 and 634 cm⁻¹ correspond to the asymmetric bending modes of P–O bonds, and a small band at 492 cm⁻¹ can be assigned to symmetric bending of P–O bonds in phosphate tetrahedra. Multiple bands observed at 1007, 1029, 1068 and 1108 cm⁻¹ correspond to the asymmetric stretching of P–O bonds in PO₄³⁻ tetrahedra. Importantly, other extra peaks were not observed in FTIR spectra, which re-confirm the monophasic nature of these phosphors.^30,31

Fig. 4 FT-IR spectra of Gd_0.93−xEu_0.07Ca_xPO₄ phosphors.

3.4. Photoluminescence (PL) properties

Photoluminescence excitation spectra (λ_em = 594 nm) of Gd_0.93Eu_0.07PO₄ phosphor prepared at 950 °C are shown in Fig. 5. From the excitation spectrum, it can be observed that a broad charge transfer band (CTB) arises due to charge transfer from O²⁻ ion to Eu³⁺ ion, situated at 246 nm, and an intense peak overlaps with CTB at 272 nm due to the f–f transition (⁸S_7/2 → ⁶I_11/2) of Gd³⁺. There are multiple sharp excitation peaks in the range of 300–550 nm arising from the Eu³⁺ and Gd³⁺ ion transitions. Out of these transitions, (⁷F₀ → ⁵I₆) at 310 nm, (⁷F₀ → ⁵H₆) at 317 nm, (⁷F₀ → ⁵D₄) at 361 nm, (⁷F₀ → ⁵G₄) at 382 nm, (⁷F₀ → ⁵L₆) at 394 nm, (⁷F₀ → ⁵D₃) at 416 nm and (⁷F₀ → ⁵D₂) at 465 nm transitions can be attributed to electronic transitions of Eu³⁺ ions.³²

Fig. 5 Photoluminescence excitation spectra (λ_em = 594 nm) of Gd_0.93Eu_0.07PO₄ phosphor prepared at 950 °C.

From Fig. S3 (ESI†), it is noticeable that the peak position of CTB of O²⁻ → Eu³⁺, situated at 246 nm, exhibits enhanced spectral intensity with a slight red shift as a function of Ca²⁺ ions co-doping concentration in Gd_0.93Eu_0.07PO₄ phosphors without any change in the peak positions of excitation spectra. The position of the CTB mainly depends on co-ordination environment of cations around the anion (O²⁻), electron affinity of anion, co-ordination number, size and charge of the cation.³³ Hence, this red shift might be reasoned to change in co-ordination environment around the O²⁻ ion with co-doping concentration.

Fig. S4 (ESI†) represents the Photoluminescence emission spectra for Gd_0.93Eu_0.07PO₄ (λ_ex = 394 nm) phosphors synthesized at 650 °C, 750 °C, 850 °C and 950 °C for 12 h. Upon excitation at 394 nm, the intense emission peaks appear between 585 to 600 nm, with maximum emission at 594 nm corresponding to magnetic-dipole transition (⁵D₀ → ⁷F₁); the weak broad emission bands between 605 and 625 nm correspond to the electric dipole transition (⁵D₀ → ⁷F₁). Other emission bands at 580, 651 and 685 to 705 nm result from the ⁵D₀ → ⁷F₀, ⁵D₀ → ⁷F₃, ⁵D₀ → ⁷F₄ transitions of Eu³⁺ activator ions. Generally, the electric–dipole transition is dependent on the host environment and the magnetic dipole transition is independent of the host environment.

From the emission spectrum it is evident that the magnetic-dipole transition (⁵D₀ → ⁷F₁) is dominant than that of the electric–dipole transition (⁵D₀ → ⁷F₂), signifying that the Eu³⁺ ions are in the inversion site symmetry.^33,34 These PL emission results show that the PL emission intensity increases with the increase in calcination temperature. This effect can be explained as follows: as the temperature increases from 650–950 °C, the extent of monoclinic Gd_0.93Eu_0.07PO₄ phosphor phase increases from cubic Gd₂O₃ phase. At 950 °C, monophasic Gd_0.93Eu_0.07PO₄ is obtained (Fig. S1, ESI†), and hence, same optimized results are reflected in PL emission studies.

The photo-luminescent emission spectra of Gd_0.93−xEu_0.07Ca_xPO₄ (0.0 ≤ y ≤ 0.07) phosphors, prepared at 950 °C for 12 h, were recorded at 394, 248, 361 and 464 nm excited wavelengths, shown in Fig. 6 and Fig. S5–S7 (ESI†), respectively. The emission features are similar to the earlier spectra (Fig. S4, ESI†), except for the magnitude of intensities for Gd_0.93−xEu_0.07Ca_xPO₄ phosphors. Fig. S8 (ESI†) shows variation of the emission intensity of ⁵D₀ → ⁷F₁ peak as a function of Ca²⁺ concentration at different excitation wavelengths. It is worth noting that upon excitation, the emission intensities increase with increasing Ca²⁺ doping concentration from 1 to 7 mol% at all four excitation wavelengths. But λ_ex = 394 nm showed highest luminescent emission intensity for ⁵D₀ → ⁷F₁ transition compared to other excitation wavelengths (248, 361 and 464 nm). Hence, PL emission spectra recorded with 394 nm excitation wavelength were used to investigate the other photo-luminescent and radiative properties. The emission intensity of Ca²⁺ co-doped phosphors was found to be enhanced 1.5 times compared to that of its parent without affecting the positions of the emission peaks.

Fig. 6 Photoluminescence emission spectra of Gd_0.93−xEu_0.07Ca_xPO₄ phosphor prepared at 950 °C for 12 h and enlarged view of ⁵D₀ → ⁷F₁ peak (inset).

This enhancement of PL emission intensity on Ca²⁺ co-doping may be reasoned the following way. An increment in energy transfer from O²⁻ ions to Eu³⁺ ions was noticed in PL excitation spectrum (Fig. S3, ESI†) when Ca²⁺ ions are introduced in Eu³⁺ activated GdPO₄ matrix. The charge to radius ratio of nine co-ordinated Ca²⁺ ions (Pauling's electronegativity for Ca²⁺ = 1.00 and effective ionic radii Ca²⁺ = 1.180 Å) is less than that of nine co-ordinated Gd³⁺ ions (electronegativity for Gd³⁺ = 1.20 and effective ionic radii for Gd³⁺ = 1.107 Å) of host, and this mismatch³⁵ creates the constricted binding between O²⁻ and Gd³⁺/Eu³⁺, which is expected to be released. Hence, doping of Ca²⁺ ions modifies the crystal field and reduces the symmetry around the Eu³⁺ ions in GdPO₄ lattice. This, in turn, helps to relax the selection rule and increases the photon transition probability, which facilitates the enhancement of charge transfer process that can be realised in CTB. Conversely, the enhancement in PL emission intensity was observed.²⁶ Moreover, co-doping of divalent calcium ions in trivalent gadolinium sites creates the charge imbalance or oxygen vacancies. Incorporation of Ca²⁺ ions at Gd³⁺ ions site effects the negative charge balance in the lattice and these negative charges can be compensated by oxygen vacancies (V_O).³⁶ These oxygen vacancies act as sensitizers and help in efficient energy transfer of absorbed photons to the activator ion.²⁴ Hence, oxygen vacancies also make a small contribution towards PL emission intensity enhancement.

3.5. Luminescence life time measurement

Fig. 7 represents the photo-luminescent decay curves of Gd_0.93−xEu_0.07Ca_xPO₄ (0.00 ≤ y ≤ 0.07) recorded at room temperature (λ_ex = 394 nm and λ_em = 594 nm). All the decay curves were fitted well to the second order exponential function, given as follows:

I_t = I₁ exp(−t/τ₁) + I₂ exp(−t/τ₂) (1)

where I₁ and I₂ and are the decay intensities at two different times, and τ₁ and τ₂ are corresponding lifetimes of excited energy states. The τ_average or τ_exp can be estimated by using the following relation:The bi-exponential decay behavior of Gd_0.93−xEu_0.07Ca_xPO₄ is mainly due to the presence of two types of active centers (Eu³⁺ ions), namely, activators present on the surface of the phosphors (surface activators), and activators present inside the phosphors (core activators). Usually, surface activators show rapid decay over core activators; hence, these phosphors exhibit bi-exponential decay nature.³⁷

Fig. 7 Fluorescence life time decays of Gd_0.93−xEu_0.07Ca_xPO₄ recorded at λ_ex = 394 nm and λ_em = 594 nm.

From Fig. 7, it is evident that the decay lifetimes increase as the Ca²⁺ co-doping increases, and the average decay life times of Gd_0.93−xEu_0.07Ca_xPO₄ (0.0 ≤ y ≤ 0.07) phosphors increase from 3.02 to 3.18 ms. This enhancement in lifetime may be due to improvement in energy transfer and radiative transitions. Moreover, the enhancement in the lifetime of the ⁵D₀ → ⁷F₁ level of Eu³⁺ ions again confirms the increase in PL emission intensity with respect to Ca²⁺ substitution.

3.6. Judd–Ofelt intensity parameters and radiative properties

The Judd–Ofelt (J–O) parameters for Gd_0.93−xEu_0.07Ca_xPO₄ (0.0 ≤ x ≤ 0.07) phosphors were evaluated by using the emission spectra of these phosphors. The magnetic and electric dipole transition peak intensities are utilized to estimate the magnetic dipole and electric dipole line strengths of Gd_0.93−xEu_0.07Ca_xPO₄ phosphors. By using estimated values of electric dipole line strengths, J–O intensity parameters Ω_λ (λ = 2, 4), radiative transition probability (A_R), radiative lifetime τ_R, non-radiative relaxation rate (W_NR), total transition probability (A_T), and theoretical photo-luminescence quantum efficiency (η) can be determined using the procedure reported in our earlier study²² and displayed in Table 1. The Ω₆ parameter cannot be estimated due to practical restrictions, and also U⁽⁶⁾ matrix, corresponding to ⁵D₀ → ⁷F₆ transitions of Eu³⁺ ions is usually weak and negligible.³⁸ The Ω₂ varies with covalency and local structural crystal field changes around the Eu³⁺ ions, and Ω₄ is less sensitive to these factors. Therefore, a lesser value of Ω₂ compared to that of Ω₄ shows less covalent nature between the Eu³⁺ and O²⁻ ligands. It was observed that as the concentration of Ca²⁺ doping increases, Ω₂ value also increased owing to the decrease in the local symmetry around Eu³⁺ ion.³⁹ Additionally, Ω₄ value also increased with the increase in the concentration of Ca²⁺ co-doping, signifying the decrease in electron density of the O²⁻ ligand.

Table 1 Calculated Judd–Ofelt intensity parameters, radiative transition probability (A_R), non-radiative relaxation rate (W_NR), total transition probability (A_T), calculated radiative (τ_R) lifetime, experimental (τ_exp) lifetime, lifetime quantum efficiency (η) and asymmetric ratio of Gd_0.93−xEu_0.07Ca_xPO₄

λ ex (nm)	Ca^2+ ion conc. (mol%)	J–O intensity parameters (×10^−20 cm^2)		A R (s^−1)	W NR (s^−1)	A T (s^−1)	τ rad (ms)	η (%)	Asymmetric ratio
		Ω 2	Ω 4
	0.00	0.80	1.73	239.66	91.47	331.13	4.17	72.38	0.50
	0.01	0.82	1.89	247.18	81.77	328.95	4.04	75.14	0.52
394	0.03	0.85	1.88	247.51	78.22	325.73	4.04	75.99	0.54
	0.05	0.91	1.96	256.28	62.19	318.47	3.90	80.47	0.57
	0.07	0.93	2.07	262.14	52.32	314.46	3.81	83.36	0.59

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The asymmetric ratio (Red/Orange ratio), i.e., ratio of the area under the electric dipole (⁵D₀ → ⁷F₂) transition to the area under the magnetic dipole (⁵D₀ → ⁷F₁) transition for Gd_0.93−xEu_0.07Ca_xPO₄ phosphors was found to be less than unity (Table 1). These results indicate that Orange emission dominates the red emission in these phosphors. As the Ca²⁺ co-doping increased, value of asymmetric ratio also increased. This increment in the ratio is due to the increased local distortion or reduced symmetry around Eu³⁺ ions compared to the non-co-doped one³⁴ and hence, intensity of the PL emission (w.r.t λ_ex = 394 nm) was enhanced (Fig. 8). Table 1 gives the comparison of τ_exp and η for Gd_0.93−xEu_0.07Ca_xPO₄ phosphors. Gd_0.86Eu_0.07Ca_0.07PO₄ shows the highest η around 83.36% and indicates possible practical utility of phosphor in solid state lighting and other display device applications.

Fig. 8 The relation between PL emission intensity and asymmetric ratio in Gd_0.93−xEu_0.07Ca_xPO₄ phosphors recorded at λ_ex = 394 nm.

The color emitted by the phosphors can be realized in terms of human eye visualization system mathematically by Commission Internationale de l’eclairage (CIE) coordinates.⁴⁰ CIE chromaticity coordinates of Gd_0.93−xEu_0.07Ca_xPO₄ phosphors can be calculated from the PL emission spectra recorded at λ_ex = 394 nm. Fig. 9 illustrates the CIE 1931 chromaticity diagram of Gd_0.93−xEu_0.07Ca_xPO₄ recorded at λ_ex = 394 nm. The x, y color coordinates of Gd_0.93−xEu_0.07Ca_xPO₄ phosphors are listed in Table 2. From Fig. 9, it is evident that the color coordinates fall in the orange-red region of the CIE diagram⁴¹ and the color emitted by a phosphor varies slightly with the concentration moving towards longer wavelength region.

Fig. 9 (a) CIE 1931 chromaticity diagram of Gd_0.93−xEu_0.07Ca_xPO₄ and Gd_0.93−xEu_0.07Li_0.05PO₄ phosphors recorded at λ_ex = 394 nm; (b) enlarged view of orange red region in CIE 1931 chromaticity diagram; photographs of (c) Gd_0.93Eu_0.07PO₄ and (d) Gd_0.93−xEu_0.07Ca_xPO₄ phosphor powders under UV light.

Table 2 CIE 1931 Chromaticity co-ordinates and CCT values of Gd_0.93−xEu_0.07Ca_xPO₄ phosphors recorded at λ_ex = 394 nm

Phosphors	CIE 1931 co-ordinates		CCT values (K)	Color purity (%)
	x	y
Gd0.93Eu0.07PO4	0.6212	0.3782	1703	98.43
Gd0.92Eu0.07Ca0.01PO4	0.6222	0.3772	1712	98.68
Gd0.90Eu0.07Ca 0.03PO4	0.6223	0.3774	1712	98.72
Gd0.88Eu0.07Ca 0.05PO4	0.6238	0.3757	1729	99.08
Gd0.86Eu0.07Ca 0.07PO4	0.6247	0.3748	1739	99.30

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Correlated color temperature (CCT) is one of the substantial factors to assess the phosphor efficiency. CCT is calculated by converting the (x, y) coordinates of the light source to (u′, v′).²² By McCamy's approximation,⁴² the CCT value can be calculated from x, y co-ordinates. The evaluated CCT values for Gd_0.93−xEu_0.07Ca_xPO₄ phosphors are in the range of 1704 to 1739 K (Table 2). Generally, CCT values below 5000 K are known to be best for production of warm white light.^43,44

The color purity of the phosphors is also one of the significant parameters to evaluate the compatible light source for practical lighting applications. Hence, color purity of these phosphors can be estimated by using the following relation:

where (x, y), (x_d, y_d) and (x_ee, y_ee) are the chromaticity coordinates of the sample point, the dominant wavelength point and equivalent energy point (0.3101, 0.3162), respectively.⁴⁵ The color purity of Gd_0.93−xEu_0.07Ca_xPO₄ phosphors was found to be in the range of 98–99%.

The estimated color purity results are tabulated in Table 2 and show that the color purity of Gd_0.93−xEu_0.07Ca_xPO₄ phosphors increased monotonically with Ca²⁺ substitution. Gd_0.93−xEu_0.07Ca_xPO₄ phosphors showed highest color purity of 99.30%, signifying that this phosphor is a possible candidate for WLEDs and other display device fabrication.

3.7. Temperature dependent photo-luminescent properties

Fig. 10(a) shows the temperature-dependent PL emission spectra of GdPO₄:Eu³⁺ (7 mol%) phosphors, measured in the temperature range of 303–533 K (λ_ex = 394 nm). These spectra clearly indicate a decrease in the emission intensity of GdPO₄:Eu³⁺ (7 mol%) phosphors by increasing the temperature because of the thermal quenching effect. The normalized PL emission intensity of GdPO₄:Eu³⁺ (7 mol%) phosphors as a function of temperature is shown in Fig. 10 (b). The PL intensity at 423 K is about 66% of the initial value at 303 K, indicating that GdPO₄:Eu³⁺ (7 mol%) phosphors possess excellent thermal stability and could serve as suitable candidates for high-power LEDs. The activation energy for the thermal quenching can be found using the following relation:where I₀ and I are the integral emission intensities at initial and measured temperatures T, respectively. A is a constant for a certain host, E_a is the activation energy for thermal quenching, and K is the Boltzmann constant (8.62 × 10⁻⁵ eV). From the plot of ln[(I₀/I)] against 1/kT, shown in Fig. 10(c), it can be clearly shown that the measured data can be linearly fitted with the slope of −0.256, suggesting that the E_a for the thermal quenching is 0.256 eV. This value is higher than the previously reported La₆Ba₄Si₆O₂₄F₂:Sm³⁺ (0.16 eV),⁴⁶ KBaPO4:Sm³⁺ (0.18 eV),⁴⁷ Sr₃La(VO₄)₃:Sm³⁺ (0.19 eV),⁴⁸ KLaSr₃(PO₄)₃F:Sm³⁺ (0.16 eV)⁴⁹ and NaY₉(SiO₄)₆O₂:Sm³⁺ (0.21 eV)⁵⁰ Sm³⁺ values for activated orange red light emitting phosphors. The relatively high activation energy value of these phosphors indicates the potential candidateship of these phosphors in the fabrication of WLEDs and optoelectronic devices.

Fig. 10 (a) The temperature-dependent emission spectra, (b) the normalized PL emission intensity as a function of temperature and (c) the plot of ln[(I₀/I)] against 1/kT of Gd_0.93−xEu_0.07Ca_xPO₄ phosphors.

4. Conclusions

Series of orange light emitting Eu³⁺/Ca²⁺ co-doped GdPO₄ phosphors were prepared by conventional solid state route. PL studies and Judd–Ofelt parameter calculations confirm that Eu³⁺/Ca²⁺ co-doped GdPO₄ phosphors exhibit enhanced PL emission and radiative properties. This enhancement of PL emission can be attributed to the size and electronegativity mismatch between the co-dopant and the host cation. CIE 1931 coordinates (0.6247, 0.3748) of 7 mol% Ca²⁺ co-doped phosphor shows highest color hue in the orange red region. Lower CCT (1739 K), excellent thermal stability and high color purity values promote the candidateship of this phosphor for practical application in fabrication of house hold WLEDs and other display devices.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors acknowledge the chemical science division, Indian Institute of Science for providing the Time resolved fluorescence spectrometer and High resolution TEM facilities. One of the authors, Pramod Halappa, is thankful to Council of Scientific and Industrial Research (CSIR), New Delhi, India for providing CSIR SRF.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8nj04372h

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